Operating Theatre Ventilation System Design

Design Considerations for Operating Theatre Ventilation Systems

There are many types of operating theatres within hospitals and healthcare facilities – all of which require a suitable “final stage of air filtration” that is dependent on facility purpose and healthcare requirements. Air filtration options for operating theatres include individual HEPA modules, laminar flow and UCV (ultra clean ventilation) systems. This article will focus on some important design considerations relating to these air filtration options within operating theatres.

The fundamental function of air filtration within an operating theatre is to remove contaminants from the air, to reduce the possibility of particulate entering a wound, and ideally to provide a protective sterile zone around the wound. The ultimate aim is to reduce the risk of infection from airborne particulates.

What Are You Trying To Achieve?

In asking “what are you trying to achieve”, there are really several questions being asked;

What types of surgeries are intended for this operating theatre?

What standards or Health Department “Operating Theatre Guidelines” do you want to comply with?

What cost have you budgeted for? How much money are you willing to spend?

More importantly;

What level of risk is the functioning body (hospital / healthcare facility) willing to take?

What is the potential financial impact on the patient and the healthcare provider with the incidence of surgical site infection (SSI) by airborne contaminants?

All too often a decision is made based on the minimum cost to meet a particular standard or guideline (for a particular type of surgery) without really considering the risk properly. Whether a new installation or a re-work of an existing theatre, an improvement on what was available previously is the minimum we must strive to achieve, with an aim to maximise the cost benefit for all.

Minimum Acceptable Requirements

Assuming that the minimum acceptable requirement for an operating theatre is to meet AS1668; this requires the supply air to the theatre to have a final HEPA grade filter, a minimum air change rate (ACH) of 20 air changes per hour and to be at a positive pressure to the surrounding areas.

HEPA filter location – All moving matter generates particles, and within an operating theatre, particles may be generated by the movement of theatre personnel, the electric motor on a piece of surgical equipment, and from airflow through the supply air duct. Particles generated within the supply air duct may include metal oxides from the duct itself, or mould / spores growing within. As such, the ideal scenario is to locate the HEPA filters as close to the supply air outlet as possible, at the terminal. This ensures any particles introduced or generated within the duct are caught by the HEPA filter just prior to the air entering the operating theatre.

Air change rate (ACH) – The air change rate is a simple calculation of the room volume x 20. This is the minimum requirement to meet standard for operating theatre ventilation but may not be sufficient to control the airborne contaminants.

Note: a supply air quantity that achieves the minimum 20 ACH, probably won’t be sufficient for a Laminar flow or UCV system.

Positive air pressure – This is the easiest part, and is a simple matter of ensuring sizing is done correctly. The sum of the return, exhaust and leakage must be less than the supply (which has a fixed minimum).

By meeting these three requirements, you ensure that;

the air supplied into the theatre is clean/and sterile (HEPA filters),

the air is provided in a quantity deemed sufficient to dilute particulates in the room to a suitably low level, based on the number of particle per cubic meter (ACH),

the air entering the theatre does so in a controlled way, and only from the HEPA filtered supply terminals (positive air pressure)

What it does not ensure is that the air introduced is done in such a way that particulate are kept away from the operating zone. This is where theatre system design is critical, as there is a possible risk of contamination by airborne contaminants which may cause SSI.

Note: the operating theatre temperature and humidity is a function of the theatre AC system capacity and is not influenced by the theatre air quality management.

Individual HEPA Modules / Laminar Flow Systems (UCVs)

Many studies discuss the advantages and disadvantages of theatre types, and a significant point raised is cost versus the reduced risk of SSI. The theatres in question generally range from low airflow, terminals (with a lower installation and maintenance cost) through to high airflow, laminar flow or UCV systems with higher installation and ongoing running costs.

There are studies that question the benefits of laminar flow and UCV over individual terminals (refer to Brandt, C article.[1]) However, when you take into consideration all studies and other factors there is a general consensus that a well-designed (downward) laminar flow / UCV system will provide a 2% reduction in SSI’s. [2],[3],[4],[5]

Primary recommendations found to reduce SSI’s include:

Correct hygiene procedures

Prophylactic antibiotics (which with MDR bacterial can be limited)

Full body suits for the theatre personnel (which is highly unlikely)

Laminar flow or UCV systems

Correct surgical technique when operating with a UCV (not leaning over the patient into the airstream onto the patient)

Individual HEPA Modules

It is possible to meet minimum acceptable requirements (through the application of HEPA filters, required air change rates and air pressures) using individual HEPA modules and a conventional theatre layout – whereby four (4) terminal HEPA modules are located around the clean zone.

Figure 2.Threshold velocity of 0.15 m/s – showing that in the rest of the room the velocity is below 0.15 m/s.

As can be seen by CFD modelling in Figure 1 and Figure 2, this provides limited areas of coverage. This method introduces supply air to the theatre functions by “dilution”; mixing clean sterile air with the particulate laden air. This proves effective in a static state, but has a balance point that can be far from ideal when the theatre is occupied and surgery is occurring.

In a static state, minimal particles are generated within the space, so providing clean sterile air through terminals will progressively dilute the particle load to an acceptable level (possibly achieving a cleanroom classification of ISO 7 or 6, or even better). However, once personnel enter the theatre and surgery begins, the rate of dilution cannot keep up with the generation of particles, and as such the particle levels within the theatre increases.

Unfortunately also due to the turbulent nature of airflow within a theatre with individual terminals, there is minimal control of where this particle laden air may end up. Although the air within the theatre is much cleaner than outside, there is still a high risk of particulate (carrying bacteria or viruses) being carried in an uncontrolled way into the operating zone and ending up in the wound.

Figure 3 and 4.Threshold velocity of 0.4 m/s – showing little uniformity of airspeed, and as such directional control, so the particles within the space are being removed via dilution rather displacement.

Laminar Flow or UCV Systems

If we take the above four (4) terminal HEPA modules; progressively make them larger and move them closer together – the coverage area and the ratio of sterile to contaminated air increases. The airflow uniformity improves and ultimately the outlets merge to become one large outlet. This then provides uniform flow of air, downward from the diffuser – and as the air slows toward the operating table, air movement also occurs outwards, away from the table. This is the start of a laminar flow system.

As a general guide, (Australian State Guidelines) a “Laminar flow” is a system with a diffuser outlet greater than 1800 x 1800mm and an “Ultra Clean Ventilation (UCV)” is a diffuser greater than 2400 x 2400mm. It should be noted individual Australian state guidelines provide values for table velocities for Laminar flows / UCVs. These values vary from state to state, and likewise these vary internationally.

Some guidelines also provide a nominal diffuser velocity of 0.35-0.41 m/s in order to achieve the required table velocity.

A well designed and applied Laminar flow / UCV provides protection to the operating clean zone in two (2) ways; (1) positive pressurisation with sterile air ensures that no contaminants can migrate into the clean zone and (2), any air contaminated from within the protected zone is rapidly displaced by clean air.

Figure 6 and 7. CFD modelling of a UCV System

Figures 6 and 7 clearly show the uniformity of airflow, down and across the operating table, with the required table velocity achieve directly above the clean zone.

The primary advantage of a laminar flow or UCV is the controlled airflow across the operating area, with sterile air sweeping across the immediate clean zone (any particulate generated in the area is swept away, and with correct operating technique the likelihood of particulate from the operating staff being swept into the clean zone and wound is reduced.

Studies suggest that this results in an approximately 2% reduction in SSI’s.2,3,4,5 2% may not seem a large percentage rate, however with the thousands of surgeries occurring every day, this small percentage certainly adds up.

Although there is some variation within the guidelines, we recommend the below airflows as a starting point for, procedures/size and airflow size:

Lower Velocity Systems (European)

The European Standard DIN 1946-4, permits lower table velocities (partly as a driver for energy consumption) and as such lower airflows. With the lower velocity, the risk of buoyancy effects and turbulence from natural heat sources (people / lighting) affecting the clean zone is increased. In an untested environment (theatre setup) this could result in a failure to meet the downward airflow requirements across the clean zone (and as such the potential for mixing of air and contamination of the wound site with particles).

To counter this (and mitigate the risk of noncompliance and potential contamination), the DIN 1946-4 standard also stipulates more stringent testing requirements to ensure that with the lower diffuser velocity; the table velocity and airflow uniformity is also being met and the clean zone is still swept in sterile (non- infectious) air.

A system that is designed to meet the European DIN1946-4, with generally lower target table velocity’s, will require more meticulous setup works and additional testing to confirm correct operation, with resulting table velocities that may still not meet Australian state guidelines. As such this is a decision that can only be made by the end user/designer, when weighing up the final risk versus any benefits that may be gained.

The Effect of Blanking Sections of a Laminar Flow System.

Figure 8 and 9 represent how spaces between individual diffusers, or how the blanking of a section of a laminar flow system can introduce an area of non-uniformity and turbulent air. This results in non-controlled air or air contaminated with particulates (such as squames or skin cells from operating staff) possibly entering the wound site and increasing the risk of SSI’s.

Airborne Particles and SSI’s

A number of technical papers and reports have been written relating to the significance of airborne particles contributing to SSI’s.

“The majority of SSIs are a result of hygiene-related factors associated with surgical personnel. With respect to bacteria transmitted to the surgical site through the air, squames or skin scales, are the primary source of transmission”.[6]

Airborne particles are found to be responsible for about 80%-90% of microbial contamination (CDC 2005).

It is generally understood that indoor air in an operating theatre may contain particulates from a number of sources (including people and processes or activities in the operating theatre), and that micro-organisms on these air particles can settle on the wound, dressings and surgical instruments and cause infections.

Reductions in hospital acquired infections can have a significant impact on improved patient outcomes and minimising the cost to the health care facility. While hygiene-related prevention is the most practiced and proven method, airborne-related contamination control offers one area that could play a much larger role. One area of ongoing discussion is the role of operating theatre ventilations systems and system design in airborne containment control to assist in the reduction of hospital acquired SSI’s.

Operating Theatre Air Quality

In 2010, Airepure undertook a review of the air quality within two operating theatre systems utilising independent industry resources; one with a traditional design for the ventilation system incorporating four terminal HEPA filters and 20 air changes per hour, and one with laminar air flow theatre ventilation with 40 air changes per hour (2.4 x 2.4m square laminar flow system with a face velocity of approx 0.4m/s).[7]

Figure 10: Traditional 4 Terminal HEPA module arrangement

Figure 11: Laminar Flow / UCV Theatre

The traditional theatre (Figure 10) showed a high level of particle contamination, both at the operating theatre table level and throughout the theatre. The tests were carried out for three traditional theatres in the same surgical department with similar results for each theatre.

The results of the laminar flow theatre (Figure 11) showed a dramatic reduction in the airborne particle contamination both at the operating theatre table level and throughout the theatre.

The assessment was carried out using a calibrated particle counter with particle counts measured and recorded in the 0.3 micron, 0.5 micron and 5 micron particle size ranges.

For both theatre ventilation systems the results were zero particle counts for all three particle sizes when the air quality was measured at the discharge from the diffusers below the HEPA filters however the results showed significant improvement in the air quality readings at the table height in the laminar flow theatre compared to the traditional turbulent flow theatre.

A summary of the results of the particle counts recorded are summarised in the following:

The most interesting observation is the rapid decline in air quality below the HEPA filters in a traditional theatre with the individual HEPA filters arrangement.

This is due to the entrainment of particles from the adjacent space. On comparison with clean room design principals, the turbulent flow arrangement would not be acceptable. High turbulence leads to pollution or contamination as well as surface areas.[8]

During the theatre observations there were numerous staff entries from the sterile corridor to set up for the next series of procedures, this had no discernible effect on the observations at the table location (zero values returned).

A well designed laminar flow/UCV system provides two protective effects: positive pressurisation, with no contaminated external entering the theatre by inflow from open doors reaches or perimeter areas can migrate to the protection zone and any air contaminated with in the protected zone is rapidly displaced by clean air from the laminar flow /UCV system.[9]

Final Thoughts

Whilst any improvement to existing operating theatre ventilation systems is an advantage, the ultimate consideration compares cost and risk. How does the installation, operation and ongoing maintenance costs of a chosen operating theatre ventilation system compare with the cost of SSI’s (patient readmission, additional care and/or surgery)?

A % improvement of SSI’s associated with a well-designed and applied Laminar flow/UCV system may seem a small percentage, but with the emergence of multi-resistant bacteria’s, this may be critical for patient and a greater cost benefit to all in the long term.

Written by Kristian Kirwin (B.ENG Mechanical) and Shannon Roger (B.Ed) for Airepure Australia, and published in Healthcare Facilities Volume 40, No 3, September 2017.

Republished in slightly edited form within Health Estate Journal, Volume 72, No 5, May 2018.

Related

A technical paper titled “Operating Theatre Ventilation System Review” was recently published in The Australian Hospital Engineer Journal over 2 issues (Dec 2014 and Mar 2015). Written by Andrew Sutherland, ASE Consulting, this paper is essential reading.

HEPA air filters play a vital role in maintaining indoor air quality (IAQ) by reducing the fine airborne particles that can harbour bacteria and viruses – critical in many hospital settings, locations including but.